This invention relates to carbon monoxide removal from a hydrocarbon reformate fuel. More specifically, it relates to apparatus and methods that use a catalytic material to adsorb carbon monoxide and an electrical current to initiate a chemical reaction between an oxidizing agent and carbon monoxide that has been adsorbed by the catalytic material, thereby regenerating the material.
The internal combustion engine found in most cars and trucks burns a hydrocarbon fuel such as diesel or gasoline to drive pistons or rotary mechanisms by the force of the expanding gas. Many electrical power plants burn fossil fuel to produce electrical energy through combustion turbines. These processes suffer from a number of limitations. They are inefficient because of the intrinsic limit of the thermodynamic principles involved. Burning of fossil fuel is oftentimes incomplete and produces harmful byproducts such as carbon monoxide, nitrogen oxides and various hydrocarbons in the emissions, which have resulted in environmental pollution. In addition, there is a growing awareness that we are rapidly depleting the non-renewable energy resources on this planet. This, in turn, has led to concerns about the reduction of energy consumption by increasing efficiency and utilizing renewable energy resources.
Fuel cells convert the chemical energy in the fuel directly into electrical energy through an electrochemical reaction. Because they do not operate on the principle of gas expansion through combustion, they do not suffer the same limitations of thermodynamic efficiency commonly found in automobile engines and steam turbines. Accordingly, it is possible for fuel cells to achieve a level of efficiency far greater than that seen in most traditional industrial processes. Additionally, fuel cells make it possible for fuel processors to use renewable forms of energy such as methanol and ethanol, thereby conserving the limited fossil fuel resources of the planet. Moreover, because of the operating environment of a fuel processor and fuel cell, hydrocarbon, nitrogen oxide and carbon monoxide emissions are negligible, approaching a zero emission state.
While there are several types of fuel cells existing in practice, this invention is targeted mainly for applications in polymer electrolyte fuel cells (PEFCs) which are also known as proton exchange membrane fuel cells (PEMFCs). A very efficient PEFC uses pure hydrogen for fuel and oxygen for an oxidant. Pure hydrogen, however, has traditionally been difficult to handle and relatively expensive to store and distribute. Consequently, attempts have been made to use hydrogen rich gas mixtures obtained from reforming of various hydrocarbon fuels. To obtain a convenient and safe source of hydrogen for the fuel cells, on-board reforming of hydrocarbon based fuels, such as gasoline and methanol, is expected to be utilized. However, these fuels usually contain nitrogen, carbon dioxide, and low levels of carbon monoxide in the range from 100's of ppm to a few percent. While the presence of carbon dioxide generally has little effect on the efficient operation of a fuel cell, even relatively low concentrations of carbon monoxide can degrade fuel cell performance. The degradation results from the carbon monoxide chemically adsorbing over the active sites in the electrode of the fuel cell. Thus, the removal of carbon monoxide from fuel has become a major concern in the advancement of PEFC technology.
Prior attempts to remove carbon monoxide from a gas mixture include a pressure swing adsorption method disclosed in Nishida et al., U.S. Pat. No. 4,743,276. They disclose a method for selectively absorbing carbon monoxide by means of Cu(l) disposed on a zeolite support, including the step of adiabatically compressing a gas mixture in the pressure range of 0.5 kg/cm.sup.2 to 7 kg/cm.sup.2. Golden et al., U.S. Pat. No. 5,531,809 disclose a vacuum swing method as a variation of the pressure swing method disclosed in Nishida. A solid absorbent is selected which physically absorbs carbon monoxide under pressure. When the pressure is reduced to the range of approximately 20 to 100 torr, the carbon monoxide is released from the solid absorbent. By cyclically repeating this process, carbon monoxide may be removed from a gas.
There are, however, multiple limitations to applying the pressure swing adsorption method to fuel cell applications. Firstly, bulky and expensive pressure resistant tanks, as well as pressure and vacuum pump apparatus, are required to carry out the process. The parasitic weight and volume of these devices make it extremely difficult to apply the pressure swing adsorption method for transportation applications such as a fuel cell power plant for an automobile. A second disadvantage of this approach is the significant power expenditure necessary to cycle the pressurization and depressurization steps. This additional power consumption will result in the reduction of overall efficiency of the fuel cell system. Yet another disadvantage of this process is that the toxic carbon monoxide released from desorption has to be converted to carbon dioxide with additional process steps and equipment.
Another prior art process has been referred to as preferential catalytic oxidation (PROX) of carbon monoxide which was documented in U.S. Pat. No. 5,271,916 by Vanderborgh et. al. In the PROX process, a small amount of pure oxygen or air is mixed into the reformate fuel before it enters a one--or multiple stage catalytic reactor. The catalyst in the reactor, which usually contains dispersed precious metals such as platinum, ruthenium, iridium, etc., preferentially reacts with carbon monoxide and oxygen to convert them to carbon dioxide. Due to the limited selectivity, however, more than a stoichiometric amount of oxygen is needed to reduce carbon monoxide to an acceptable level. Also, the excess oxygen will oxidize the hydrogen in the reformate fuel. Even with the PROX process, the concentration of CO in the reformate stream is often still significantly higher than the desirable level for sustainable PEFC operation. Furthermore, the carbon dioxide in the reformate may be converted to carbon monoxide through a reversed water-gas shift reaction inside of the fuel cell.
To further eliminate residual carbon monoxide that escapes from the pretreatment or forms from the reversed water-gas-shift reaction inside of the fuel cell, a direct oxygen injection to fuel cell method was developed. For example, Gottesfeld, U.S. Pat. No. 4,910,099 discloses a method of injecting a stream of oxygen or air into the hydrogen fuel so as to oxidize the carbon monoxide. Pow et al., U.S. Pat. No. 5,316,747 disclose a similar means of eliminating carbon monoxide directly by introducing pure oxygen or an oxygen containing gas along the latter portion of a reaction chamber in an isothermal reactor in the presence of a catalyst that enhances the oxidation of the carbon monoxide. Wilkinson et al., U.S. Pat. No. 5,482,680 disclose the removal of carbon monoxide from a hydrogen fuel for a fuel cell by means of introducing a hydrogen rich reactant stream into a passageway having an inlet, an outlet and a catalyst that enhances the oxidation of carbon monoxide; introducing a first oxygen containing gas stream into the hydrogen rich reactant stream through a first port along the passage way, thereby oxidizing some of the carbon monoxide within the reactant stream; and introducing a second oxygen containing gas at a subsequent point, further oxidizing the remaining carbon monoxide. Wilkinson et al., U.S. Pat. No. 5,432,021 similarly oxidize carbon monoxide to carbon dioxide by means of an oxygen containing gas introduced into a hydrogen rich reactant stream in the presence of an unspecified catalyst.
There are several significant limitations for the PROX process and oxygen injection process. One of these limitations is the parasitic consumption of hydrogen. Due to the limited selectivity, the oxidant injected into a hydrogen rich fuel is always higher than the stoichiometric amount necessary for oxidizing the carbon monoxide. The unreacted oxygen will consume hydrogen in the stream to therefore reduce the overall fuel efficiency. Another significant limitation of these methods is their poor tolerance towards the variation of CO input level in the reformate. To minimize the parasitic hydrogen loss, the oxygen to CO ratio has to be kept at a relatively low level in both approaches. Yet, the CO input level often varies as the result of change of the fuel cell power output and, thus, the reformate throughput. It is difficult to constantly match the CO input level with the oxygen level in a dynamic environment. Consequently, unreacted CO will exceed the fuel cell tolerance level, leading to poor performance. Yet another limitation of these two approaches is the concern over safety. The oxygen to hydrogen ratio in the mixture has to be strictly controlled below the explosion threshold.
Another prior art process for removing carbon monoxide involves membrane separation, whereby the hydrogen in the reformate can be separated by a metallic membrane. For example, R. E. Buxbaum, U.S. Pat. No. 5,215,729 discloses a palladium based metallic membrane which provides the selectivity for hydrogen separation up to 100%. Therefore, it could remove carbon monoxide and other components from hydrogen which is the fuel for the PEFC. Although highly selective, the process has several disadvantages. Since it uses precious metal as membrane material, it is expensive. Furthermore, the reformate has to be pressurized to facilitate the separation process which results in parasitic power loss and equipment complexity.
Methanization is another prior art process to remove carbon monoxide through the catalytic reaction of carbon monoxide with hydrogen to form methane. An example of this method is given by Fleming et. al., U.S. Pat. No. 3,884,838. Methane does not have a detrimental impact and is regarded as non-reactive in the fuel cell. The methanation reaction, however, requires hydrogen as a reactant and, therefore, increases the parasitic consumption of the fuel for the fuel cell. Furthermore, under the methanation condition, not only carbon monoxide but also carbon dioxide participates in the reactions. The reaction of carbon dioxide with hydrogen generates carbon monoxide through chemical equilibrium. It is therefore difficult to reduce the carbon monoxide level to the desirable limit for PEFC operation.
Because of the sensitive nature of fuel cells, it is vital that carbon monoxide removal approaches 100% efficiency. In addition to the process limitations, such as cost, excess volume and weight, system complexity and high parasitic hydrogen consumption for the above mentioned methods, there is another common shortcoming, i. e., slow response during the cold start of the fuel cell power plant. Most of these approaches need the system to reach a certain temperature before they are operable, which often represents an undesirable delay between start-up and normal operation.
Therefore, there is a need for a method of removing carbon monoxide from a hydrocarbon reformate that is highly efficient, displays enhanced tolerance to carbon monoxide concentration swings, reduces parasitic hydrogen consumption, eliminates venting of carbon monoxide into the atmosphere, can be operated simply and economically, and can operate at the temperature and pressures of a fuel cell, as well as during the start-up mode.